"Tu as un chargeur mais tu n'as pas un adaptateur."
I do not agree with you. Here the question of definite or indefinite article is not an issue, since the French sentence uses indefinite articles for both nouns (chargeur - adaptateur). The issue is with the use of "pas un" becoming "de/d' ".
In French, you will say "il n'a pas un adaptateur" means "not even one adapter", suggesting that he were supposed to have plenty and has none.
Sitesurf, it seems to me that you're right. After a further refresh of high school grammar I agree with your point of how the sentence uses indefinite articles. Interestingly, google translates the sentence as above, though if you enter "Je n'ai pas d'adaptateur" it translates as I don't have an adapter.